As the power output of ducted fan gas turbine engines rises, so do their structural dimensions, including the external diameter of the outer cowling of the fan duct. It follows that unless the dimensions are kept to the minimun possible, performance penalties in the form of increased installed drag will be generated.
A controlling factor in fan cowl design is the relationship between the highlight dimension (measured across the air intake lip in a plane which contacts the most upstream points on the lip) and the cowl maximum dimension (measured laterally of the engine axis). This relationship is defined by the size and change in the free stream tube flow into the engine. By the stream tube flow is meant that ambient air upstream of the engine air intake and extending to the air intake itself.
If the free stream tube size is maintained at its maximum size over the whole of the engine operating cycle, it will result in a cowl maximum dimension which is closer to the highlight dimension, thus effectively reducing the fan cowl profile and therefore the installed drag. This is achieved by virtue of that ambodiment air which flows over the exterior of the cowl, doing so with reduced change in direction.
Maintenance of the free stream tube size is achieved by varying the thrust controaruring given flight regimes.
The present invention seeks to provide a ducted fan gas turbine engine including an improved means for thrust control and resultant free stream tube size control.